The first possible choristoderan trackway from the Lower Cretaceous Daegu Formation of South Korea and its implications on choristoderan locomotion

Abstract

Here we report a new quadrupedal trackway found in the Lower Cretaceous Daegu Formation (Albian) in the vicinity of Ulsan Metropolitan City, South Korea, in 2018. A total of nine manus-pes imprints show a strong heteropodous quadrupedal trackway (length ratio is 1:3.36). Both manus and pes tracks are pentadactyl with claw marks. The manus prints rotate distinctly outward while the pes prints are nearly parallel to the direction of travel. The functional axis in manus and pes imprints suggests that the trackmaker moved along the medial side during the stroke progressions (entaxonic), indicating weight support on the inner side of the limbs. There is an indication of webbing between the pedal digits. These new tracks are assigned to Novapes ulsanensis, n. ichnogen., n. ichnosp., which are well-matched not only with foot skeletons and body size of Monjurosuchus but also the fossil record of choristoderes in East Asia, thereby N. ulsanensis could be made by a monjurosuchid-like choristoderan and represent the first possible choristoderan trackway from Asia. N. ulsanensis also suggests that semi-aquatic choristoderans were capable of walking semi-erect when moving on the ground with a similar locomotion pattern to that of crocodilians on land. South Korea has become globally famous for various tetrapod footprints from Cretaceous strata 1 , among which some clades such as frogs 2 , birds 3 and mammals 4 have been proved for their existences only with ichnological evidence. Recently, a new quadrupedal trackway with a pronounced heteropody (sensu stricto) 5 was discovered in the Daegu Formation (Albian, Lower Cretaceous), Ulsan City, which consists of nine manus-pes sets (Fig. 1). Both manus and pes are pentadactyl with sharp claw marks. They look different from any known quadrupedal dinosaur footprints because there is a clear indication of webbing between the pedal digits. During the Mesozoic, the webbed footprints have been occasionally reported in various quadrupedal animals such as freshwater turtles 6 , amphibians 7 , crocodyliforms 8 , and pterosaurs 9. However, new footprints differ from their tracks in morphology and appear to belong to that of other clade. Therefore, the purpose of this paper is to describe the new type of tracks and to investigate the possible trackmaker by comparing their ichnological features with skeletal ones. In addition, the trackmaker's locomotory posture on land was inferred from the trackway pattern. Results Systematic ichnology. Ichnogenus Novapes ichnogen. nov. (Figs. 2, 3). Type species. Novapes ulsanensis ichnogen. et ichnosp. nov., by monotypy and designation herein. Diagnosis. Quadrupedal tracks with a pronounced heteropody; pentadactyl manus impression with claw marks and semi-symmetrical outline, wider than longer; divergence between digit I and V imprints ranges 180° to 210°; the digit IV imprint slightly longer than digit II; entaxonic; pentadactyl pes impression with claw marks and asymmetrical outline (i.e., lateral digits are more developed), longer than wide; webbing bet...